In a number of fields of technology it is desirable to be able to provide a flow of water (or other liquid) for a predetermined but adjustable period of time. For example, in garden and other irrigation systems it is desirable to provide a timer which permits flow for a certain period and then closes a shut-off valve. Another use of timers is in conjunction with semi-automatic dishwashing machines which are becoming more popular. In such machines water pressure is used to rotate a rack, and water sprays clean the dishes on the rack. The time for which the washer is run depends on the type of dishes being washed i.e. cups and saucers require a short time and greasy plates a longer time. An adjustable timer has great utility in conjunction with such machines.
Conventionally, timers incorporate a clock-work mechanism which is wound up or, in more sophisticated forms, electrical or electronic systems. These later forms require a power source, which it is not always convenient to provide.